Most elementary science teachers would like to give their students opportunities to do science. The "Next Generation Science Standards" and "A Framework for K-12 Science Education" (NGSS Lead States 2013; NRC 2012) make this goal explicit by requiring that students learn how to engage in the practices of science. Consequently, achieving the goals of these new standards will require teachers to assess whether students' thinking demonstrates engagement in science practices. Unfortunately, the NGSS provide little guidance for this assessment, which is challenging for prospective teachers who may have limited experience with seeing students do science. In this article, the authors explain three criteria they use with preservice teachers to help them assess students' "doing science" by focusing on the substance of students' thinking processes. The article includes examples from teacher education programs to demonstrate how teachers use the criteria to assess students' ideas and plan future pedagogical choices.